1. Field of the Invention
This invention relates to a molded or formed article of a thermoplastic saturated norbornene polymer provided with a hard coat large having excellent adhesion and bond strength and high hardness, and to a process for the production of said molded or formed article. A molded or formed article produced by the process of this invention is particularly suitable as an optical material.
2. Related Art
It is conventinal practice to use polycarbonate (PC) and polymethyl methacrylate (PMMA) mainly as an optical transparent plastic molding material, e.g. for an optical disk substrate and a plastic lens. Since, however, PC has high birefringence, and since PMMA has high water absorption, it is increasingly difficult to meet with advancing technological requirements.
In recent years, a thermoplastic saturated norbornene polymer such as a hydrogenation product of a polymer formed by ring opening polymerization of a norbornene monomer or an adduct type polymer of a norbornene monomer with ethylene has attracted attention as an optical plastic molding material for an optical disk, and the like (JP-A-60-26024, JP-A-1-24826, JP-A-63-31752, EP 303,246, EP 317,262, U.S. Pat. No. 4,614,778, JP-A-61-120816). The thermoplastic saturated norbornene polymer is a very useful material for optical use due to its low birefringence and excellent transparency, heat resistance, water absorption resistance, etc. Further, since said polymer is also excellent in strength, water resistance, electric insulation, solvent resistance and resistance to chemicals such as acids and alkalis, it is also useful as an electrically insulating material and a humidity-resistant packing material such as a container and a film in addition to the optical use.
However, a molded or formed article produced from said material has a problem in that a hard coat layer peels off from the molded or formed article after being cured due to poor wettability with a general-purpose hard coating agent and inadequate adhesion. And, there has not been known any molded or formed article of a thermoplastic saturated norbornene polymer which is provided with a hard coat layer having a bond strength, measured by a crosscut adhesion test, of not less than 90% and a surface hardness, measured by a pencil hardness test, of not less than 2H, nor has there been known any process for the production thereof.
In general, a plastic molded or formed article is sometimes required to have a high surface hardness depending upon use. For example, optical disks such as a compact disk and a laser disk come in direct contact with the hands of its user when used, and therefore, when its surface is flawed due to contact with other substance, an error takes place in reading a stored memory content. In addition, in the optical use and the field of a packing container, the flaw on the molded article surface is undesirable since the transparency is degraded. In order to avoid such a flaw, the surface of a molded or formed article is generally required to have a pencil hardness of not less than 3H. A molded article produced from a thermoplastic saturated norbornene polymer usually has a surface hardness of about H, and such a molded article is required to be improved to some extent.
For these reasons, the molded or formed article surface is sometimes modified by forming a hard coat layer on the surface. Further, the hard coat layer formed on a transparent plastic molded or formed article can maintain the transparency and fine appearance of plastic, and can impart the molded or formed article with excellent resistance to chemicals and contaminants. Therefore, the formation of such a hard coat layer leads to a widening of the application range of the plastic, and it is possible to develop a versatile use of a molded article as a substitute for glass and as an automobile part, an electric and electronic part, a construction material, a furniture material, and the like.
Such a hard coat layer is required to bond strongly to a substrate, and is usually tested by a crosscut adhesion test, in which the hard coat layer is cut lengthwise and widthwise eleven times each with a sharp-edged cutter at intervals of 1 mm to form 100 squares which individually have an area of 1 mm.times.1 mm, and a certain adhesive tape is firmly pressed thereon and speedily peeled off at right angles. The bond strength by this test is evaluated by counting the number of squares which are not peeled off. A molded article is required to have, in general, a bond strength of not less than 90%, particularly preferably of 100%.
Meanwhile, the general-purpose hard coating agents can be generally classified into a silicone-based hard coating agent and an organic hard coating agent.
This silicone-based hard coating agent is a partial hydrolysis product of a silane compound, and requires curing treatment under heat at a relatively high temperature of 120.degree. C. for about one hour. As a result, a plastic substrate is thermally deformed slightly. Therefore, the silicone-based hard coating agent is not suitable for a plastic molded or formed article of which accuracy is required.
On the other hand, the organic hard coating agent is grouped into a type in which a melamine, alkyd, urethane or acrylic coating composition is cured under heat and the other type in which a polyfunctional acrylic coating composition is cured under UV irradiation. The former type is easy in handling. Its defect is, however, that it is inferior in hardness and weatherability. The latter is excellent in hardness and productivity, and causes little thermal influence on a resin due to UV curing. Therefore, a UV curable polyfunctional acylic hard coating agent is suitable as a hard coating agent for a transparent plastic molded article.
The UV curable polyfunctional acrylic hard coating agent is usually a product which contains a polyfunctional acrylic monomer and/or oligomer, a photopolymerization initiator and other additive, and which is with or without a solvent (thinner) as a diluent.
In PC and PMMA which are general-purpose transparent plastic materials, alcohol solvents such as isopropyl alcohol, glycol ether solvents such as ethylene glycol monomethyl ether, ester solvents such as ethyl acetate and butyl acetate and ketone solvents such as acetone and methyl ethyl ketone are used as a thinner for a UV curable hard coating agent.
However, even if the UV curable hard coating agent using the above thinner is applied to a molded or formed article formed of the thermoplastic saturated norbornene polymer, the resultant hard coat layer is easily peeled off since it has poor adhesion with the molded or formed article after being cured. The reason therefor is as follows: The thermoplastic saturated norbornene polymer is one of the olefin resins and the surface of a molded or formed article thereof has poor wettability and high chemical resistance. Therefore, neither of the polymer and the hard coating agent is mutually diffused into another's phase through the interface thereof, and molecular entanglement hardly occurs. As described above, the thermoplastic saturated norbornene polymer is one of the materials which are as difficult to bond or treat with a chemical as olefin resins such as polyethylene and polypropylene are.
Therefore, there have not been known any thermoplastic saturated norbornene polymer molded articles provided with a hard coat layer having excellent adhesion and high surface hardness and an effective process for the production thereof.